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    Re: Tides by bearing of the moon
    From: Hewitt Schlereth
    Date: 2009 Apr 7, 11:28 -0400

    As I recall luni-tidal intervals were included in Bowditch till the
    30's as another column in the list of maritime positions.
    
    Growing up on Long Island Sound and sailing in New England generally,
    I found it useful and even included it in a book on coastal
    navigation.
    
    In comparing this lunar method with the published tables, I noted that
    there were often noticeable differences between what the table would
    say and what I actually observed on the water. I figured this was
    because the tables are generated by a computer which could not know
    what the wind or barometer were going to be on the predicted day and
    time. Both wind and barometric pressure affect the tide.
    
    Living here in the US Virgin Islands I've dabbled at it, but the tides
    here are not diurnal as they are on the US East Coast but more like
    what I've heard of the tides around England. This thread may inspire
    me to have a more earnest go at it.
    
    Hewitt
    
    
    On 4/7/09, George Huxtable  wrote:
    >
    >  Thanks to D Walden for pointing us toward Archibald Patoun, author of a
    >  treatise in navigation which was published in Glasgow in 1730 and remained
    >  in print, in several editions, over the next half-century. I must admit to
    >  never even having heard of his name before. Perhaps he was better known
    >  North of the border.
    >
    >  His book has been fully digitised and available at-
    >  http://books.google.com/books?id=8yEB2eQCxg0C&printsec=titlepage&source=gbs_summary_r&cad=0#PPA87,M1
    >
    >  The passage that was brought to our attention was the way of stating tide
    >  predictions according to the compass bearing of the Moon. I hadn#t realised
    >  that it had survived so late as the mid-18th century.
    >
    >  This curious procedure had by than a long history of a couple of centuries,
    >  documented first in tide-maps by the Breton, Brouscon. As a VERY  rough
    >  approximation, high water at a particular port precedes or follows the
    >  meridian passage of the Moon by a fixed interval, and that is the assumption
    >  that it's based on. In the days when mariners carried no timepiece, and were
    >  lucky to have even a sandglass, it was easier to think of the tide as
    >  relating to the passage of the Moon in terms of the Moon's bearing rather
    >  than a time interval. After all, in mediaeval times, they had been
    >  accustomed to the day, dawn to dusk, being divided into 12 hours, which
    >  could differ greatly from the 12 hours of the night, and which changed with
    >  the seasons. So allowing for passage of time in absolute-hours wasn't an
    >  easy concept.
    >
    >  But the whole business is rather a fiction. It assumes that the direction of
    >  the Moon changes by 15 degrees per hour, and this was supposedly confirmed
    >  by look-up tables. That would be roughly true if the Moon was viewed from
    >  the North Pole, but much less so when viewed from lower latitudes. Patoun's
    >  list relates to ports in Northwest Europe, but if viewed from the tropics
    >  (as an extreme example), the Moon would be nearly East all morning, quickly
    >  changing to nearly West, around noon.
    >
    >  Because there are two tides each day, Patoun always lists two opposite
    >  bearings on which the two high-tides occur. From the coast of Flanders, for
    >  example, he gives the Moon's bearing as South, and North. The Moon's bearing
    >  being South is clear enough, when meridian transit occurs, but of course the
    >  Moon can never have a bearing of North. Indeed, the whole concept of "Moon's
    >  bearing" is a nonsense when the Moon is below the horizon. Nor is the
    >  concept particularly useful when the Moon is invisible because it's nearly
    >  new. Nor, indeed, when it's overcast, even.
    >
    >  Anyway, the time-shift, between Moon meridian passage and high water, alters
    >  considerably as the phase of the Moon changes, because of the changing phase
    >  differences of the gravity-pulls, from Moon and Sun. So the whole business
    >  could only be rough-and-ready, at its best.
    >
    >  Rather more logical in concept was the definition of "tidal establishment"
    >  of a port, which applied only to times of full Moon and new Moon, and
    >  referred then to the clock-time of high water.
    >
    >  As for Patoun's book as a whole, it seems very long-winded, making heavy
    >  weather of concepts that appear trivial to us today, such as noon latitude
    >  observations, and with much irrelevant material about ancient calendar
    >  systems. It carries useful traverse tables and five-figure log trig tables.
    >  Anyway, it's been interesting to see it for the first time.
    >
    >  George.
    >
    >  contact George Huxtable, at  george{at}hux.me.uk
    >  or at +44 1865 820222 (from UK, 01865 820222)
    >  or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    >
    >
    >
    >  >
    >
    
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